Light-emitting diode (LED) technology has advanced to the point where LEDs can be used as energy efficient replacements for conventional incandescent and/or fluorescent light sources. One application where LEDs have been employed is in ambient lighting systems using white and/or colored (e.g., red, green and blue) LEDs. Like incandescent and fluorescent light sources, the average luminous flux of an LED's output is controlled by the average current through the device. Unlike incandescent and fluorescent light sources, however, LEDs can be switched on and off almost instantaneously. As a result, their luminous flux can be controlled by switching circuits that switch the device current between two current states to achieve a desired average current corresponding to a desired luminous flux. This approach can also be used to control the relative intensities of red, green, and blue (RGB) LED sources (or any other set of colored LED sources) in ambient lighting systems that mix colored LEDs in different ratios to achieve a desired color.
One approach to LED switching is described in U.S. Pat. Nos. 6,016,038 and 6,150,774 of Meuller et al. These patents describe the control of different LEDs with square waves of uniform frequency but independent duty cycles, where the square wave frequency is uniform and the different duty cycles represent variations in the width of the square wave pulses. U.S. Pat. Nos. 6,016,038 and 6,150,774 describe this as pulse width modulation (PWM). This type of control signal has high spectral content at the uniform frequency and its odd harmonics, which can cause electromagnetic interference (EMI) to sensitive devices, components, circuits, and systems nearby.
U.S. Pat. Nos. 6,016,038 and 6,150,774 also describe a conventional networked illumination system that utilizes a DMX512 protocol to address network data to multiple individually addressed microcontrollers from a central network controller. Using the DMX512 protocol, the relative luminous flux of each individual color in a light source is transmitted from a lighting controller to a light source, as illustrated in FIG. 1A.
In solid-state (LED) lighting, the luminous flux output of each LED at a given operating current decreases as the junction temperature of the LED increases. LED junction temperature can increase due to power dissipation in the LED and/or increases in ambient temperature. This effect, illustrated in the curves of FIG. 1B, can create both luminous flux errors and errors in color mixing because the magnitude of the effect is different for LEDs of different colors.
Another temperature effect in LEDs is a shift of the dominant wavelength of an LED as the junction temperature of the LED changes. Typically, the dominant wavelength increases as junction temperature increases, causing a red shift. This effect can cause additional color distortion independent of the luminous flux effects.
Another effect in LED lighting networks is LED aging. In general, the luminous flux of an LED decreases with accumulated operating time. The rate of decrease is different for different color LEDs and is affected by the operating current and temperature of the LED. This effect can cause luminous flux errors and color distortion independent of the other effects mentioned above.
In one conventional system, three individual colors are mixed using a three-color mixing equation. The three-color mixing equation is used to solve for the three intensity levels needed from the three light sources to create a mixed color. In another conventional system, four or more colors can be mixed; however, instead of solving for the four intensity levels with an equation, a controller of the conventional system uses a look-up table to determine the intensity levels for the four or more colors, as illustrated in the conventional system of FIG. 1C that mixes four colors. The conventional system of FIG. 1C includes a processor that receives a mixed-color request. Instead of deterministically solving for the dimming values of the four light sources, the processor accesses dimming values in the look-up table stored in memory, and sends the looked up dimming values to the dimming circuitry and light sources. This type of conventional system is limited in the number of allowable color coordinate inputs. The number of allowable color coordinate inputs is limited by the size of available memory (e.g., read-only memory (ROM) or Flash memory) in which the look-up table is stored. By increasing the number of allowable color coordinate inputs in the look-up table, the size of memory also increases. As such, there are fewer unique colors that can be mixed using this conventional system. Also, it makes temperature and optical feedback to account for light source variances unfeasible. This is because for each feedback variable, another dimension of look-up tables must be added in memory. Therefore, the memory use increases exponentially as information feedback is implemented.
When using only three light sources in a conventional color mixing system, the color rendering index (CRI) may be limited. The CRI increases when more bands of light frequencies, which are used to create the mixed color, are available. For example, if only red, green, and blue light sources are available, only red, green, and blue wavelengths are present. On the other hand, if more than three unique colors are used to create the same mixed color, the CRI increases. However, as described above, since look-up tables limit the number of unique mixed colors that can be created, if four or more colors are mixed to create the mixed color using look-up tables, the amount of memory needed increases. For instance, in a four-color system, which has an 8-bit resolution of intensity control, over four billion unique colors can be created. It would take a lot of memory to store dimming values for four billion colors (e.g., more than 16 gigabytes (GB)) in a look-up table. Also, if only a selected number of colors are used in the look-up table, the total possible number of unique colors that can be created is limited. In the example above, if a 1 MB memory is used, roughly 250,000 unique colors out of the four billion possible can be created.